1. Field of the Invention
The present invention relates to a method and a system for displaying binary dot matrix data in order to represent various kinds of images, e.g. characters, as a gray-level image. More particularly, the present invention relates to a display method and a display system for reducing line flicker caused by interlaced scanning.
2. Description of the Related Art
In recent years, more and more systems, e.g. a home shopping system, utilizing a household TV (television) receiver as an information terminal have been developed. A TV receiver used for such systems displays character information, e.g. an operation menu, on the display screen thereof. In general, a character font is constructed as a binary dot matrix. That is to say, the character is displayed on the screen of a TV receiver as a dot matrix consisting of two colors of black (as a background color) and white (as a foreground color), for example. In the case of displaying a monochrome character on the screen on a TV receiver, however, the following problems arise.
When a character or the like showing a large vertical brightness transition is displayed on the screen, a flickering state, e.g. a so-called line flicker, occurs on the screen, thereby making a viewer feel some annoyance. This phenomena results from the fact that the scanning system for a TV receiver is an interlaced scanning system. When a character having a high-resolution is displayed, the colors and/or especially the brightness differs in the adjacent lines. In other words, the brightness transition is very large in a "vertical direction" in such a case. Here, "vertical direction" means a direction perpendicular to scanning lines of the TV receiver's cathode-ray tube. Therefore, only a low-resolution display employing a large character font may be realized by a conventional system due to flicker.
A flicker is caused by interlaced scanning using a frequency of 30 Hz per frame (i.e., a field frequency of 60 Hz). If the scanning is conducted by using a frequency twice as high as an ordinary frequency per frame, i.e. the frequency of 60 Hz per frame, then the flicker becomes imperceptible to human's eyesight. In order to accomplish this duplication, the scanning is required to be conducted either by a non-interlaced scanning system, or by a frequency-duplicated interlaced scanning system with a vertical frequency duplicated to 120 Hz. According to the latter method, however, a scanning-system switching function is required to be equipped for a display system, and the circuit is required to operate in a larger bandwidth, so that the cost necessary for such a display system increases considerably.
Even in the case of conducting an interlaced scanning, the flicker may be reduced by reducing the vertical brightness transition. For example, a flicker never occurs in a displayed image of a ordinary TV broadcasting. In order to reduce a flicker, it is necessary to suppress the brightness transition between adjacent lines to a small level. The brightness in a plurality of adjacent lines is generally smoothed by processing the plurality of lines utilizing a vertical low-pass filtering technique when a raster scanning is conducted. According to this method, however, a line buffer memory for storing the data corresponding to a plurality of lines in order to process a single line and a memory controller thereof are required, so that the circuit becomes disadvantageously large. To be sure, this method may reduce the flicker, but the sharpness is sacrificed over the entire picture displayed on the screen. Accordingly, in view of the prevention of the degradation of the display quality, the processing is preferably conducted with respect to the character display region alone.
U.S. Pat. No. 4,454,506 entitled "Method and Circuitry for Reducing Flicker in Symbol Displays" discloses such a technique conducted with respect to the character display region alone. This patent is related to a method and a circuitry for reducing a flicker as being applied to the display conducted by a raster scanning while reading character fonts from a font read-only memory (ROM), e.g. a character display terminal. More specifically, according to this technique, first, the brightness difference between the dot of the scan line being currently scanned and that of the scan lines adjacent to the scan line being currently scanned is detected. Next, if the brightness difference is at a predetermined level or more, the signal levels of the dot on the adjacent scan lines are controlled, thereby reducing the brightness transition. According to this technique, however, the signal to be applied to the scan lines adjacent to the current scan line is changed. Specifically, the character font itself being scanned is output not in a distorted state. However, the regions adjacent to the character font in a direction perpendicular to the scanning direction are affected. As a result, the size of the character font is changed.
Japanese Patent Publication No. 4-78230 entitled "Character Broadcasting Receiving System" discloses another processing technique conducted with respect to the character display region alone. According to this receiving system, first, a character font represented based on the binary data is filtered by using plural kinds of aperture patterns. Thereafter, the binary data of the filtered character font is synthesized, thereby generating a gray-level font data. According to this method, however, surrounding nine pixels are required to be processed in order to generate a multi-level data for one pixel. Therefore, in the case where such a processing is conducted sequentially, the resulting speed for processing becomes slow. On the other hand, in the case where such a processing is conducted in parallel, the size of the circuit is required to be large.
According to other methods, a character font is held beforehand in gray levels instead of the binary data in order to reduce a flicker. Such a technique is presented by John E. Warnock: Proceedings of the Conference SIGGRAPH '80, The Display of Characters Using Grey Level Sample Arrays, July 1980, pp. 302-307; and by Nicholas Negroponte: Proceeding, Society for Information Display, 1980. In these methods, however, a TV receiver is also required to have character fonts in gray levels. Therefore, the amount of the information of the character fonts to be stored in a memory becomes disadvantageously large, so that a hardware for displaying such fonts cannot be realized at a low cost.